Anterior Ethmoidal Artery: A Computed Tomography Analysis and New Classifications

Abstract

Objective  to determine the anterior ethmoidal artery (AEA) anatomy and variations by computed tomography (CT) in adult and their relations to and presents new AEA classifications.

Methods  One hundred and fifty paranasal CT scans (300 sides) were included. Axial images were acquired with multiplanar reformates to obtain delicate details in coronal and sagittal planes.

Results  One hundred and forty-four AEAs canal (48%), 293 AEAs foramen (97.7%), and 229 AEAs sulcus could be detected (76.3%). The mean AEA intranasal length was 6.7 ± 1.27 mm (range: 4.24–10.6 mm). The mean angle between AEA and lamina papyracea was 105.49 ± 9.28 degrees (range: 76.41–129.76 degrees). Of them, 95.8% AEAs had an angle with lamina >90 degrees, while 4.2% had angle <90 degrees. The mean angle between AEA and lateral lamella of cribriform plate was 103.95 ± 13.08 degrees (range: 65.57–141.36 degrees). Of them, 87.5% AEAs had an angle >90 degrees and 12.5% had an angle <90 degrees. The mean distance between AEA and skull base was 1.37 ± 1.98 mm (range: 0–8.35 mm). The AEA types in relation to skull base was type 1 (0–2 mm from skull base; 64.6%), type 2 (2–4 mm; 22.2%), type 3 (4–6 mm; 11.1%), and type 4 (>6 mm; 2.1%). The mean distance between the AEA and frontal sinus ostium was 9.17 ± 4.72 mm (range: 0–25.36 mm). AEA classification according to distance from AEA to frontal sinus ostium was 17.4% type 1 (<5 mm), 41.7% type 2 (5–10 mm), 31.9% type 3 (10–15 mm), and 9% type 4 (>15 mm).

Conclusion  Provided AEA details improve surgeons' awareness of AEA variations in the endoscopic field and can help residents in training.

Introduction

E ndoscopic sinus surgery (ESS) is nowadays one of the most otorhinolaryngologically performed surgeries. ^{1 2 3} With the advancement of the endoscopic technologies, instrumentation, and the proper imaging modality, ESS has been applied beyond the nasal cavity and the paranasal sinuses extended to the orbit and skull base. ^{4 5 6}

The anterior ethmoidal artery (AEA) runs through the roof of anterior ethmoidal sinuses traveling from the orbit to the anterior cranial fossa. AEA is a major anatomical landmark that is vulnerable to accidental injury during surgery on the anterior ethmoidal sinus and skull base. Its injury may result in profuse epistaxis, intraorbital bleeding, and retroorbital hematoma that can lead to blindness (if not decompressed within approximately an hour), or in rare cases, intracranial bleeding. ⁷

The AEA as a landmark is important for accessing the frontal sinus and in skull base surgery. ^{8 9 10} Additionally, AEA identification serves to define and treat some cases of severe epistaxis. ¹¹ However, its deep position, multiple variations, and complicated relations with adjacent important structures make it a high-risk area for the surgeon. AEA position relative to the ethmoidal roof is very variable that the artery is vulnerable to injury during ESS. ^{12 13}

Proper imaging study of the sinonasal anatomical variations is one of the guide tools in the way to get a safe and effective ESS. ^{14 15}

Preoperative knowledge of the course of the AEA within the ethmoid sinuses assists the surgeons to avoid its injury during ESS to get a safe surgical procedure. ¹⁶ Moreover, the anterior ethmoid sulcus in the lateral lamella of the cribriform plate creates the weakest point of the anterior skull base, where cerebrospinal fluid (CSF) leaks may occur. ^{12 17}

Computed tomography (CT) is of paramount importance not only to evaluate the sinonasal disease but also to characterize the anatomy of the paranasal sinuses ^{18 19} that varies significantly even between both sides in the same individual. ^{20 21}

Gotwald et al ²² used a coronal CT analysis to investigate anatomical landmarks for locating the AEA, and found that the notch in the medial wall of the orbit (anterior ethmoidal foramen) and the focal funneling in the olfactory fossa (anterior ethmoidal groove) were good references for identifying the position and the orientation of the AEA within the ethmoid sinus.

To our knowledge, no previous studies had focused on thorough radiological analysis of the AEA location in the surgical trajectory relevant to other anatomical structures specially the angle between the artery and the lamina, the angle between the artery and the lateral lamella, and the distance between the artery and the frontal recess. Therefore, we had conducted this work to codify by CT the relation between the AEA and the orbit, the lateral lamella, frontal recess, and the skull base. This study tried to provide precise, easily reported objective assessment of the AEA.

Methods

This retrospective analysis was made on 150 CTs of the paranasal sinuses (300 sides) at Otorhinolaryngology department, Zagazig University Hospitals and Radio Diagnosis Departments and Benha University Hospitals during the period between December 2015 and August 2018.

Informed written consent was signed by all patients to share in the study after explanation of its purposes.

Exclusion criteria were patients younger than 12 years, history of surgery or trauma in the paranasal sinuses or the skull base, and congenital anomalies, malignancies, and/or fibroosseous lesions of the paranasal sinus.

CT examinations were performed for all included patients with a 64-slice CT scan (Light speed volume VCT, GE medical system, Milwaukee, Wisconsin, United States). The protocol of 64-slice multi detector computed tomography (MDCT) was performed with a 0.625-mm detector width, a 1.5-mm section width and a 0.5-mm interval reconstruction.

Axial images were performed covering the paranasal sinuses. The patient was examined in supine position and the beam was parallel to the hard palate. The cuts began from the hard palate to the top of frontal sinus (glabela), using 130 KV and 150 mA/sec, with scan time 1.5 seconds, wide window (window widths approximately 1,300–2,000 and window levels about −80 to −200). Examinations were done with bone window setting of 3,000 HU, centered at 300 HU. High-resolution algorithm was used for enhancement of the fine bony details.

Multiplaner reconstructions with delicate detail in coronal and sagittal planes were got for all patients at a dedicated postprocessing workstation (Advantage Windows Volume share 4.5, GE Medical System, Milwaukee, Wisconsin, United States). Films were reviewed in routine standardized fashion to not miss small details.

AEA position evaluation was performed in all patients along the axial, coronal, and sagittal planes.

Following Souza et al, ¹⁰ anatomical landmarks used to identify the AEA in coronal sections were (1) the bony notch on the medial orbital wall, which corresponds to the anterior ethmoidal foramen and the bony sulcus on the lateral wall of the olfactory fossa, which corresponds to the anterior ethmoidal sulcus. The length of AEA was measured ( Fig. 1 ).

Fig. 1.

( A ) Coronal CT images in two different cases, showing anterior ethmoidal artery (AEA) canal (arrowheads), along its course through anterior ethmoidal cells and AEA foramen (arrows), which appear as a bony notch on medial wall of orbits. ( B ) Showing Anterior ethmoidal sulcus (arrows): bony sulcus on lateral walls of olfactory fossae. CT, computed tomography.

Also coronal CT sections were used to assess the cribriform plate (CP) and provide information on the AEA course in the ethmoidal roof. The distance between the midpoint of the AEA and the roof of the ethmoid sinus, the angle between the artery and lamina papyracea, and the angle between the artery and the lateral lamella of the cribriform plate were measured in the coronal planes ( Figs. 2 3 4 ).

Fig. 2.

Coronal CT shows types of AEA in relation to its distance from skull base (ethmoidal roof). Image 1: type 1, distance is 0 (AEA passes through skull base). Image 2: type 1 at left side; distance is 0 and type 2 at right side; distance = 2.65 mm. Image 3: type 3 in left side. Image 4: type 2 in right side and type 1 in left side. Image 5: type 3 in right side and type 4 in left side. AEA, anterior ethmoidal artery; CT, computed tomography.

Fig. 3.

Sagittal CT shows types of AEA in relation to posterior edge of frontal sinus ostium. Images 1, 2: type 1; distance is 0 in image 1. Image 3: type 2. Image 4: type 3. Image 5: type 4. AEA, anterior ethmoidal artery; CT, computed tomography.

Fig. 4.

measured angles between AEA and lamina paperatea ( A ) and angle of AEA and lateral lamella of cribriform plate of ethmoid ( B ). AEA, anterior ethmoidal artery.

Distance between AEA and the skull base (ethmoidal roof) was measured and classified into four types ( Fig. 2 ) as follows:

1. Type 1: distance between AEA and skull base is 0 (AEA passes through or adherent to the skull base) to 2 mm.

2. Type 2: AEA is 2 to 4 mm down the skull base.

3. Type 3: AEA is 4 to 6 mm down the skull base.

4. Type 4: AEA is more than 6 mm down the skull base.

Distance between the AEA and posterior edge of frontal sinus ostium was measured at sagittal cuts between posterior edge of the frontal sinus ostium and the AEA, if both are at the same horizontal level. While if the AEA canal and frontal sinus ostium are at different levels, a vertical line drawn at posterior edge of frontal sinus ostium and the distance to the AEA canal is then measured.

Identification of the AEA canals in sagittal plane was guided by identification of AEA canal in coronal and axial planes, then, the AEA canal was identified in the sagittal plane using reference lines in the corresponding coronal and axial planes. In Sagittal plane, the AEA canal appears as a round structure (end on) in the roof of the ethmoid sinuses. But in coronal and axial planes, the AEA canal appears as a tubular structure with oblique course in the roof of anterior ethmoid sinuses.

AEA relation to frontal sinus ostium was categorized into four types ( Fig. 3 ) mentioned below:.

1. Type 1: AEA is up to 5 mm behind the posterior edge of frontal sinus ostium.

2. Type 2: AEA is 5 to 10 mm posterior to posterior edge of frontal sinus ostium.

3. Type 3: AEA is 10 to 15 mm posterior to posterior edge of frontal sinus ostium.

4. Type 4: AEA is > 15 mm posterior to posterior edge of frontal sinus ostium.

All the observations were performed independently by two readers. If an anatomical structure was present on more than one image on the film, the observer selected the image which best captured the anatomy. For the anterior landmarks, the single frame which best represented the anatomy was chosen, taking both sides into account. When both observers identified a landmark but chose a different image, they reviewed the scans together to reach a consensus.

Statistical analysis was conducted using the SPSS statistical software package (version 25; SPSS, Inc., Chicago, Illinois, United States). p-Value of <0.05 was considered statistically significant.

Results

A total of 150 CTs (300 sides) were included in the current work; 103 males (68.7%) and 47 females (31.3%). The mean age was 33.63 ± 10.2 years (range: 18–66 years).

AEA canal was seen in 144 sides (48%) and could not be detected in 156 sides (52%). AEA foramen was seen in 293 sides (97.7%) and was not detected in 7 sides (2.3%). AEA sulcus was seen in 229 sides (76.3%) and was not detected in 71 sides (23.7%; Table 1 ).

Table 1. Measurements of AEA and its relation to skull base, frontal sinus ostium, lamina papyracea, and lateral lamella of CP.

		Total	Right sides	Left sides	Males	Females
Number of patients		150	150	150	103	47
AEA visualization (%)		144 (48)	70 (46.67)	74 (49.33)	101 (49)	43 (45.74)
Intranasal length of AEA (mm)	Mean ± SD	6.7 ± 1.27	6.72 ± 1.29	6.67 ± 1.26	6.8 ± 1.34	6.45 ± 1.05
	Range	4.24–10.6	4.35–10.6	4.24–10.6	4.32–10.6	4.24–8.52
	p -Value ( t -test)		0.7396 ( t  = 0.3327) NS		0.9438 ( t  = 0.0706) NS
Angle between AEA and lamina (degrees)	Mean ± SD	105.49 ± 9.28	103.77 ± 8.46	107.13 ± 9.77	105.42 ± 7.1	105.66 ± 7.71
	Range	76.41–129.76	83.89–123.95	76.41–129.76	76.41–129.76	94.26–128
	p -Value ( t -test)		0.002 ( t  = 3.1198) S		0.7837 ( t  = 0.2748) NS
Angle between AEA and lateral lamella of CP (degrees)	Mean ± SD	103.95 ± 13.08	105.69 ± 13.25	102.31 ± 12.79	103.61 ± 14.2	104.76 ± 10.07
	Range	65.57–141.36	76.63–137.32	65.57–141.36	65.57–141.36	78.4–123.81
	p -Value ( t -test)		0.0284 ( t  = 2.2024) S		0.4286 ( t  = 0.7927) NS
Distance from AEA to skull base (mm)	Mean ± SD	1.37 ± 1.98	1.47 ± 2.11	1.26 ± 1.85	1.46 ± 1.95	1.13 ± 2.05
	Range	0–8.35	0–8.35	0–6.58	0–8.24	0–8.35
	p -Value ( t -test)		0.3699 ( t  = 0.898) NS		0.1627 ( t  = 1.3996) NS
Distance from AEA to frontal sinus ostium (mm)	Mean ± SD	9.17 ± 4.72	9.13 ± 5.02	9.21 ± 4.47	9.21 ± 4.93	9.09 ± 4.27
	Range	0–25.36	0–25.36	0–21.71	0–25.36	0–16.88
	p -value ( t -test)		0.8865 ( t  = 0.1428) NS		0.8254 ( p  = 0.2208) NS

Abbreviations: AEA, anterior ethmoidal artery; CP, cribriform plate; NS, nonsignificant; S, significant; SD, standard deviation.

The mean intranasal length of the AEA (AEA-L) in coronal CT was 6.7 ± 1.27 mm with a range of 4.24 to 10.6 mm (6.72 ± 1.29 mm with a range of 4.35 to 10.6 mm at the right sides and 6.67 ± 1.26 mm with a range of 4.24 to 10.6 mm at the left sides). The mean length in male was 6.8 ± 1.34 mm (range: 4.32–10.6 mm) and in female was 6.45 ± 1.05 mm (range: 4.24–8.52; Table 1 ).

The mean angle between AEA and lamina papyracea in coronal CT was 105.49 ± 9.28 degrees with a range of 76.41 to 129.76 degrees (103.77 ± 8.46 degrees with a range of 83.89 to 123.95 degrees at right sides and 107.13 ± 9.77 degrees with a range of 76.41 to 129.76 degrees at left sides). While the mean angle in males was 105.42 ± 7.1 degrees (range: 76.41–129.76 degrees) and in females was 105.66 ± 7.71 degrees (range: 94.26–128). No significance difference between either sides or sexes ( p  = 0.7837; Table 1 ).

Noting that 138 of the 144 seen AEAs (95.8%) had an angle with lamina of more than 90 degrees while 6 AEAs (4.2%) had angle less than 90 degrees( Tables 1 and 2 , Fig. 4 ). Thus, AEA is mostly but not always passing superiorly from the orbit (laterally) to the cribriform area (medially). So, AEA is mostly at a lower level laterally than at CP.

Table 2. Grading of AEA in relation to skull base.

Distance of AEA from skull base		Type 1 (0–2 mm)	Type 2 (2–4 mm)	Type 3 (4–6 mm)	Type 4 (> 6 mm)	p -Value ( Q 2 )
Number		93	32	16	3
Percentage		64.6	22.2	11.1	2.1
Sides	Right	45	14	9	2	1.07 NS (0.78432)
	Left	48	18	7	1

Abbreviations: AEA, anterior ethmoidal artery; Q ² , chi-square test; NS, nonsignificant.

The mean angle between AEA and lateral lamella of the cribriform plate in coronal CT was 103.95 ± 13.08 degrees with a range of 65.57 to 141.36 degrees (105.69 ± 13.25 degrees with a range of 76.63 to 137.32 degrees at right side and 102.31 ± 12.79 with a range of 65.57 to 141.36 degrees at left sides). While the mean angle in male was 103.61 ± 14.2 degrees (range: 65.57–141.36 degrees) and in female was 104.76 ± 10.07 degrees (range: 78.4–123.81 degrees; Table 1 ). It was noted that 126/144 AEAs (87.5%) had an angle > 90 degrees and 6/144 AEAs (12.5%) had an angle < 90 degrees.

The mean distance between AEA and skull base (ethmoidal roof) was 1.37 ± 1.98 mm with a range of 0 to 8.35 mm (1.47 ± 2.11 mm with a range of 0–8.35 mm at right side and 1.26 ± 1.85 mm with a range of 0–6.58 mm at left sides). While the mean distance in male subjects was 1.46 ± 1.95 mm (range: 0.8- 24) and in female subjects was 1.13 ± 2.05 mm (range: 0.8–3.5; Table 1 , Fig. 2 ).

The most commonly found AEA type in relation to skull base was type 1 (0–2 mm from the skull base) in 93/144 AEA (64.6%). It is important to note that in 90/144 (62.5%) (90/93 of type-1 AEA, 96.8%), the AEA was passing through or adherent to the skull base with distance between AEA and skull base equal 0. Type-2 AEA (2–4 mm) was reported in 32/144 AEA (22.2%). While type-3 AEA (4–6 mm from skull base), type-4 AEA > 6 mm was reported in 16/144 (11.1%) and 3/144 (2.1%), respectively ( Table 3 , Fig. 2 ).

Table 3. Angle between AEA and lamina and lateral lamella of CP of the ethmoidal bone.

	Angle between AEA and lamina		Angle between AEA and lateral lamella of CP
	<90 degrees	>90 degrees	<90 degrees	>90 degrees
Number	6	138	18	126
Percentage	4.2	95.8	12.5	87.5

Abbreviations: AEA, anterior ethmoidal artery; CP, cribriform plate.

The mean distance between the AEA and frontal sinus ostium in sagittal cuts was 9.17 ± 4.72 mm with a range of 0 to 25.36 mm (9.13 ± 5.02 mm with a range of 0–25.36 mm at right side and 9.21 ± 4.47 mm with a range of 0–21.71 mm at left sides). While the mean distance in male patients was 9.21 ± 4.93 mm (range: 0–25.36 mm) and in female patients was 9.09 ± 4.27 mm (range: 0–16.88 mm; Table 1 , Fig. 3 ).

As regard distance between AEA and frontal sinus ostium, type-1 (0–5 mm) was detected in 25/144 AEA (17.4%), type 2 (5–10 mm) in 60/144 AEA (41.7%), type 3 (10–15 mm) in 46/144 AEA (31.9%), and type 4 (>15 mm) in 13/144 AEA (9%; Table 4 , Fig. 3 ). So, most reported AEA (73.6%) were types 2 and 3 with distance from frontal sinus ostium ranged between 5 and 15 mm.

Table 4. Types of AEA relation to frontal sinus ostium.

Distance of AEA from frontal ostium		Type 1 (0–5 mm)	Type 2 (5–10 mm)	Type 3 (10–15 mm)	Type 4 (>15 mm)	p -Value ( Q 2 )
Number		25	60	46	13
Percentage		17.4	41.7	31.9	9
Sides	Right	15	27	21	7	1.9 NS (0.59)
	Left	10	33	25	6

Abbreviations: AEA, anterior ethmoidal artery; Q ² , chi-square test; NS, nonsignificant.

In our study, supraorbital pneumatization was seen in 48/300 sides (16%). On evaluating the AEA in relation to presence of supraorbital pneumatization, we found that presence of supraorbital pneumatization significantly increase the distance between the AEA and frontal sinus ostium, the distance between AEA and skull base and the angle between lamina papyracea and AEA ( p  < 0.001). Presence of the supraorbital cell did not lead to significant difference in the angle between AEA and lateral lamella of the cribriform plate ( p  = 0.1476; Table 5 ).

Table 5. Relation of presence of supraorbital pneumatization to different AEA measurements.

Measurements	Supraorbital pneumatization		t -Test	p -Value
	Present (48 sides)	Absent (252 sides)
Distance between the AEA and frontal sinus ostium (mm)	12.4829 ± 5.19987	8.4227 ± 4.27663	5.8132	< 0.001 HS
Angle between lamina and AEA (degree)	110.8664 ± 11.2401	104.1978 ± 8.2876	4.8014	< 0.001 HS
Angle between AEA and lateral lamella of CP (degree)	101.6021 ± 19.1693	104.5224 ± 11.17374	1.4519	0.1476 NS
Distance of AEA from skull base (mm)	3.5968 ± 1.31778	0.8287 ± 1.31778	13.3383	< 0.001 HS

Abbreviations: AEA, anterior ethmoidal artery; CP, cribriform plate; HS, highly significant; NS, Nonsignificant.

Discussion

Accurate localization of the AEA is highly crucial during basic and advanced ESS because of its importance as an anatomical reference point for anterior skull base and frontal recess. ^{4 23} But there is a wide variation in the course of the AEA within the ethmoid sinus ^{24 25} and during ESS, polypi and/or thickened mucosa is often encountered in the ethmoid sinus. Thus, the AEA, particularly, when devoid of its bony canal, is difficult to be distinguished from the ethmoidal lesions. In such common situation, the CT detailed descriptions of AEA represent the main guide to identify and protect the artery.

AEA injury was reported during ESS, indicating that it is a difficult task for surgeons to identify the AEA course. ^{26 27} AEA injury may result in major negative sequelae including sever epistaxis, retroorbital hematoma, blindness, cerebrospinal fluid leak, and/or intracranial bleeding.

Most of the published studies for locating the AEA utilized endoscopic measurements and landmarks. ^{10 28 29 30} But few papers described AEA using CT ^{22 31 32} with no definite description for the angles between the artery and the orbit, as well as between the artery and the olfactory fossa.

Moreover, up till now, there is no definite classification to determine the relationship between the AEA and the skull base or the frontal sinus ostium. That is why in the current study, we tried to add a detailed CT description of the AEA and create new classifications to precisely localize the AEA with its relations in a common informative language between surgeons and radiologists.

In our study, the AEA canal was found in 48% of the patients that is slightly more than findings of Başak et al ³³ as they identified the anterior ethmoidal canal in 43% of cases that underwent a coronal plane CT and also of Souza et al ¹⁰ who found the AEA in 41% of the exams

In the current study, the anterior ethmoidal foramen was detected in 97.7%, while the anterior ethmoidal sulcus was found in 76.3% of cases and these findings were in accordance with Souza et al ¹⁰ who reported the anterior ethmoidal sulcus in 98% of their exams and Mcdonald et al ³⁴ detected AEA the anterior ethmoidal sulcus in 100% of their studied CT scans. Gotwald et al ²² reported that the AEA sulcus was found in 84% of their CT exams.

Regarding the intranasal length of the AEA, we found that the length ranged between 4.24 and 10.6 mm with a mean of 6.7 ± 1.27 mm that is near to average length that was detected by Araujo Filho et al (5.2 mm). ¹² While Yang et al ³⁵ found that the average length was 8.4 ± 1.5 mm and Yenigun et al ³⁶ found the length ranged between 6 and 20 mm. This difference in the ratio may be due to that we measured the straight distance, the difference in the size of the sample and/or the racial difference.

We found that most (95.8%) angle between AEA and lateral lamella was obtuse (>90 degrees). So, even though, AEA mostly pass obliquely in upward direction from lateral to medial, sometimes, the AEA (4.2%) courses in the opposite direction passing downward from lamina to cribriform plate.

In our study, we radiologically classified the AEA position from the skull base and frontal recess.

Lannoy-Penisson et al ³¹ classified the course of the AEA in relation to skull base into three grades as follows: grade I (artery included in the roof), grade II (artery running under the roof and considered as prominent), and grade III (artery distant from the ethmoidal roof). Similar categorizations were suggested by Yang et al, ³⁵ Cankal et al, ³⁷ and Moon et al. ⁹ But this classification is nonprecise and subjective and it does not categorize its distance from skull base that we found that it has a wide variation from 0 to 8.35 mm with different liability of injury during ESS.

Thus, we presented a new classification to describe the accurate relation of AEA level to the skull base that depends on precise CT measurements in living patients. In most cases, the AEA was found to be type 1 (0–2 mm from skull base) in 64.6%, and type 2 (2–4 mm) in 22.2%. In addition, we found that commonly (62.5%), the AEA was passing through or adherent to the skull base with distance between AEA and skull base equal to 0. Similar finding was reported by Moon et al ⁹ (85.7%), and Başak et al ³² (57%). On the other side, there is a difference between our study and that was done by Poteet et al ³⁸ and Yenigun et al ³⁶ who found that AEA located below the skull base in 25.7 and 53.2%, respectively. This difference may be due to racial difference or difference in the sample size. The AEA is more vulnerable to injury when it descend more below skull base in ethmoid sinus, the AEA types 3 and 4 are more risky during ESS.

In addition, since a considerable number of candidates for endonasal surgery have an AEA below the skull base (that was found in our study to reach up to 8.35 mm), it is important to assess this preoperatively using the standard CT imaging to avoid complications. ³⁸ Thus our classification providing new AEA types that helps to precisely predict the more risky AEA in the roof of ethmoidal cells during ESS.

The mean distance between the AEA and the skull base in the current study was 1.37 ± 1.98 mm (range: 0–8.35 mm) that is near to mean distance measured by Ko et al study ³⁹ (1.65 ± 1.9, with range: 0–6.7 mm). On the other hand, Simmen et al ⁷ and Poteet et al ³⁸ found that the artery was lying 3.7 mm (range: 1–8 mm) away from the skull base.

Another new classification to determine the relation of AEA to the frontal sinus ostium was also presented in the current study and showing that the most commonly reported grading were type 2 (5–10 mm) posterior to the posterior end of the frontal sinus ostium which was found in 41.7% of the examined cases.

The mean distances was 9.17 ± 4.72 mm which was in accord with the study of Simmen et al, ⁷ as they found that average distance between the AEA and the posterior wall of the frontal recess was 11 mm, and de Notaris et al ⁴⁰ reported the distance between the AEA and the anterior margin of the frontal recess as 10 mm. Our data were not different from the data obtained by Ko et al ³⁹ who found that the mean distance between the frontal recess and the AEA was 8.58 ± 5.56 mm (range: 0–22.6 mm). However because most AEA lied adherent to skull base, measuring to posterior border of frontal sinus ostium could be more reliable. In addition, frontal recess is a wide area and not definite location as the posterior border of frontal sinus ostium.

The grading systems presented in the current study assist surgeons to expect the AEA location and course in the surgical field particularly after revising the patient CT and this will help to compare with other cases.

In our study, the mean angle between the AEA and lamina papyracea was 105.49 ± 9.28 degrees, while Yang et al ³⁵ measured the angle from the superior side and registered it to be 60.5 ± 16.4 degrees. We registered that range of this angle was wide (76.41–129.76 degrees) and 4.2% had angle <90 degrees. Therefore, AEA is mostly but not always passing obliquely in upward direction from the orbit (laterally) to lateral lamella of cribriform plate (medially). We detected the mean angle between AEA and lateral lamella of the cribriform plate to be 103.95 ± 13.08 degrees with also a wide range (65.57–141.36 degrees) and with 12.5% had an angle <90 degrees depending on angle with lamina papyracea and the inclination of the cribriform plate. So studying of these angles in individual patient CT is important before and during surgery.

In our study, supraorbital pneumatization was seen in 48/300 sides (16%), while Souza et al ¹⁰ found supraorbital pneumatization in 35%, and Yenigun et al ³⁶ found it 40.2% of their cases. In our study, we found that there is a significant difference between the distance between the AEA and the frontal sinus ostium in case of pneumatized and nonpneumatized supra orbital ethmoidal air cells. The distance increase with the increase of pneumatization which can aid the surgeon to be more cautious during dealing with nonpneumatized supra orbital cell because the artery may be closer to the frontal sinus ostium than other pneumatized cases.

The introduction of navigation systems has definitely helped in targeting, challenging, and difficult areas during ESS. However, it should not preclude our anatomical knowledge and attention to different variations. Moreover, unfortunately, the navigator systems are not universally available and, in many centers, is not currently part of the setup for routine cases.

Finally, differences in AEA localization and classification were definitely there all the time even before the description of such variations. However, many surgeons started to see and focus on these variations only after getting the knowledge about it. Attention to the AEA location may similarly assist surgeons and residents to take a safe and effective step forward in performing closely related endoscopic work. So adding these measurements to preoperative checklist is valuable particularly in patients when dealing with pathology where the AEA is at risk or when targeting AEA, such as in cases of epistaxis. Moreover, the presented measurement, angles, and grading in current work assist in building up a common language on describing the AEA.

However, more studies are required to evaluate the relationship between different AEA grades and its injury incidence.

This information can be of great help in preoperative evaluation and operative training, and can also provide a base for future studies to confirm AEA positions associated with higher risk of injury or residual disease, which, we believe that, will be useful to otolaryngologists undertaking ESS.

Conclusion

In conclusion, to ensure the safety and efficiency of paranasal sinus surgery, it is essential for preoperative evaluation of the course of AEA. The current work updates the knowledge about the position of AEA from a CT perspective to improve surgeons' and radiologists awareness of AEA location in the endoscopic field in the way for optimum and safe surgery with least AEA injury and avoid residual disease.